1. Field of the Invention
The present invention relates to a pump having an improved fluid passage, and more particularly to a pump having an outer casing which houses a pump section or a motor.
2. Description of the Related Art
There have heretofore been known pumps having an outer casing which houses a pump or a motor. For example, a full-circumferential-flow pump disclosed in Japanese laid-open patent publication No. 6-10890 includes an outer casing of sheet metal which encloses a motor therein.
The outer casing of such a pump holds a fluid being handled on its inner surface and also houses a pump or a motor for protecting the same. A sealing member is disposed on the inner surface of the outer casing for preventing a fluid under discharge pressure from leaking into a region under suction pressure. This structure is well suited to pumps which handle a simple fluid flow therein. Specifically, the main flow of a fluid which is being handled by such a pump flows only in one direction in the outer casing after the fluid is introduced into the outer casing until it is discharged out of the outer casing. Therefore, the pump operates highly efficiently without causing any undue pressure loss.
Furthermore, because the outer casing is of a relatively simple shape, it can easily be produced by pressing sheet metal.
However, the principles of the pump, which makes only the inner surface of the outer casing hold a fluid being handled, have resulted in a limitation posed on various structural possibilities. For example, if a balanced multistage pump were to have a fluid passage from a preceding stage to a subsequent stage within an outer casing, then the pump would be of a highly complicated structure, which would make it impossible to manufacture the pump as an actual product. Moreover, if a vertical multistage full-circumferential-flow pump of the normal type, rather than the balanced type, were arranged to discharge a fluid from a lower portion of an outer casing after the fluid has sufficiently cooled the motor, then it would be necessary to provide an annular fluid passage having a large passage area around the motor. Such an annular fluid passage would be undesirable as it would increase the outside diameter of the outer casing.
Further, there has heretofore been known a full-circumferential-flow double-suction-type pump which comprises a cylindrical outer motor frame disposed around the stator of a motor, an outer cylinder defining an annular space between the outer cylinder and an outer circumferential surface of the cylindrical outer motor frame, and laterally spaced pump sections mounted on respective opposite ends of the shaft of the motor for introducing a fluid being handled into the annular space.
In the known full-circumferential-flow double-suction-type pump, a fluid drawn in from a suction port flows into the pump section in which the fluid is introduced into respective impellers. The fluid flows discharged from the impellers then flow into the annular space between the outer cylinder and the cylindrical outer motor frame, and are combined with each other in the annular space. The combined fluid flow is then discharged from a discharge port defined in the outer cylinder.
The full-circumferential-flow double-suction-type pump is effective in canceling out thrust loads developed by the fluid and providing a suction capability particularly when the pump is operated at a high speed. However, since the pump is of the double suction type, it is not suitable for use as a pump for pumping a fluid at a very low flow rate. One effective way of realizing a centrifugal pump for pumping a fluid at a very low flow rate is to reduce the width of blades of an impeller in the pump. If the width of blades is reduced, however, the efficiency of the pump is lowered, and the impeller is subject to the danger of becoming clogged with foreign matter. In addition, a double-suction-type pump as a pump for pumping a fluid at a very low flow rate is more disadvantageous than a single-suction-type pump because the amount of fluid that is pumped by the double-suction-type pump is the sum of amounts of fluid discharged from both impellers thereof.